Treatment baths of various types are widely used for purposes of providing an aesthetically pleasing or protective finish upon an article. Typical of such baths are those employed in treating metals. There are numerous varieties of such bath compositions, and reagents therefor used in the treatment of metal articles. For example, oxidizing or nitriding baths are frequently used to improve the hardness and/or color characteristics of fabricated metal articles. Problems arise however, in the use of such baths because of nonuniformity of reaction of the metal surface therewith.
The instant invention, as will be apparent from the discussion and description which follow, is applicable to a wide variety of reagent baths; however, for purposes of illustration, it will be described primarily with reference to heat treating baths of the type utilized in conjunction with processes for the nitriding, carbonitriding or oxidizing of the surfaces of ferrous articles. It has previously been known to treat ferrous articles via various vapor phase or molten salt bath processes to produce a hard coating thereupon. For example, a bath of molten cyanide-containing salts may be employed to create a hard nitride or carbonitride case and similarly, various proprietary gaseous atmospheres including mixtures of carbon monoxide, hydrogen, nitrogen, ammonia and the like are employed at various temperatures to similarly treat workpiece surfaces.
While such treatments are in wide commercial use, they by themselves are not fully adequate for all metal finishing needs insofar as problems with corrosion and quality of appearance frequently occur. Coatings produced by various of the molten salts processes tend to be rather porous and consequently manifest poor corrosion resistance. As a consequence, such parts are treated by immersion in a rust inhibitor bath so as to fill the pores thereof in a process generally referred to as "oiling out." Articles treated by the gas phase process have a lower degree of porosity and hence are not fully amenable to an oiling out process, however they do still to have sufficient porosity to create problems of corrosion. Additionally, articles treated by either of said processes often manifest a poor physical appearance insofar as the coatings may appear mottled or otherwise unappealing. In an attempt to approve the appearance of such articles, resort is often had to oxidizing baths. Such baths typically include molten nitrate or nitrite salts and optionally caustic compounds therein and function to convert an outer layer of the metallic article to black iron oxide (Fe.sub.3 O.sub.4)
While there are various processes for blackening ferrous articles, all suffer from common inadequacies. Oftentimes the reaction between the oxidizing reagent and the ferrous article is not uniform, consequently a mottled or blotched appearance results. In other instances, an undesirable residue, referred to as "smut," is formed upon the article. The smut, is generally believed to be a mixture of amorphous iron oxide and nitride, and aside from being aesthetically unappealing, tends to rub off and stain items in contact with the oxidized surface.
Accordingly, it will be appreciated that there is a need for a process which can reduce the formation of mottling or smut in such oxidation processes. It will also be appreciated that problems resultant from non-uniform reaction and/or generation of unwanted deposits can also occur in other types of treatment processes and accordingly it would be similarly desirable to eliminate such problems.
The present invention provides an improved reagent bath which promotes uniform reaction and eliminates smut formation. According to the present invention, a reagent bath includes therein an abrasive material agitated with respect to the article being treated. It has been found that the abrasive interacts with the surface being treated so as to renew the reactive centers thereupon, remove loosely adherent material, and remove deviant morphologies and other unwanted reaction products. As a result, the reagent is free to react optimally with the surface. This synergy between the abrasive and the reagent results in superior coating formation.
The present invention has great utility in various processes for the heat treating of metallic surfaces and may be readily extended to various other coating or surface conversion processes where unwanted by-products, deviant morphologies, and surface deposits interfere with the formation of a uniform surface layer. These and other features and advantages of the present invention will be readily apparent from the drawings, discussions, and descriptions which follow.